The invention relates to a test bench for carrying out aerodynamic measurements on vehicles. The test bench has at least one contact surface for detecting the weight of the vehicle, a running belt for simulating the roadway moving under the vehicle, and at least one measuring device for determining the forces acting upon the vehicle. The invention also relates to an aerodynamic measuring method for vehicles comprising the steps of detecting the vehicle weight on at least one contact surface, simulating the roadway moving under the vehicle using a running belt, and determining forces acting upon the vehicle.
Test benches of the above-mentioned type are used in the vehicle industry, particularly in the case of series-produced vehicles and racing sports vehicles, for measuring aerodynamic drag forces in a wind tunnel.
In the case of such test benches, so-called single-belt systems are known which have external force-detecting devices for measuring forces parallel to the roadway plane in a vehicle fixing system, particularly laterally beside a road wheel of the vehicle. For measuring lift forces, several load cells are integrated at the road wheels in a running belt table, and the load cells measure the wheel load through the running belt. Furthermore, so-called five-belt systems are known which have four wheel running belts integrated in a wind tunnel with one running belt arranged there-between for simulating the moving roadway.
In the case of the known single-belt systems, the aerodynamic drag forces are determined together with high rolling friction forces by the external measuring and fixing devices. However, rolling friction forces can be determined only with limited precision using calibration measurements. Under certain circumstances, aerodynamic side forces are determined incorrectly because they are partially transmitted to the running belt and the force values at the measuring and fixing devices are thereby reduced. On the whole, the single-belt systems have weaknesses with respect to the reproducibility of the aerodynamic longitudinal and side forces and result in considerable testing expenditures in order to statistically confirm the test results.
Known five-belt systems simulate the roadway moving under the vehicle by a comparatively narrow center running belt and the four wheel running belts. Flow disturbances arise in the case of this simulation technique at the running belt edges of the center running belt, which results in a partial disturbance of the flow of air into the wheel area of the vehicle measured. The weaknesses of the five-belt system during the roadway simulation can, as a rule, still be tolerated in the case of series-produced vehicles, which typically have a large clearance above the road surface and wheels integrated in the vehicle body. However, in the case of racing sports vehicles with a small road clearance, simulation errors may lead to insufficient results.
It is an object of the invention to provide a test bench and a method for carrying out aerodynamic measurements on vehicles by which the above-mentioned disadvantages are overcome and the roadway moving under the vehicle can be correctly simulated in a comparatively cost-effective and easily implementable manner.
According to the invention, this object is achieved by use of a test bench, wherein the running belt for simulating the roadway simultaneously creates at least one contact surface for detecting the vehicle weight. The running belt is arranged on a weighing plate, which is disposed on a bearing device relative to a stationary environment, and at least one measuring device determines the forces acting upon the vehicle based on the movement of the weighing plate relative to the stationary environment. This object is also achieved by an aerodynamic measuring method comprising the steps of constructing the at least one contact surface with the running belt for simulating the roadway moved under the vehicle, supporting the entire running belt on a weighing plate disposed relative to a stationary environment, and determining forces acting upon the vehicle using the movement of the weighing plate relative to the stationary environment.
In the case of the test bench according to the invention, a comparatively large running belt is mounted on a weighing plate and, as a whole, is movably disposed with respect to a stationary environment. The movable weighing plate is coupled with at least one measuring device through which the forces which occur between the weighing plate and the stationary environment during a test can be measured. The forces determined in this manner correspond to the aerodynamic drag forces on the vehicle during the test.
In comparison to a single-belt system, according to the invention, interfering rolling friction forces are compensated by the running belt and the running belt drive because the running belt is supported on or integrated with a weighing plate. Aerodynamic longitudinal and side forces are transmitted from the vehicle by way of the running belt to the weighing plate. According to the invention, the forces can easily be determined relative to the stationary environment of the weighing plate.
Interfering air friction forces on the running belt surface do not occur according to the invention because only one running belt is used and, as a result, all flow edges between running belts are avoided. Furthermore, air friction forces on the running belt surface can be excluded from the force measurement in that the same running belt speed and air speed are selected.
According to the invention, the aerodynamic forces resulting from a mounting device can likewise be omitted by using a brace such as thin cables or bracing rods whereby the aerodynamic forces can be determined by force measurements under wind pressure without the vehicle.
The invention advantageously combines the comparatively realistic simulation of the single-belt system with the reproducibility of aerodynamic forces found in a five-belt system.
As an advantageous further development of the test bench according to the invention for aerodynamic measurements, using the bearing device, the weighing plate can be displaced relative to the stationary environment, preferably in a floating manner. As an alternative, or in addition to a displacing movement, the weighing plate with its running belt arranged thereon or therein can advantageously be set or suspended on swivelable supports. The bearing device is adapted to provide an essentially frictionless movement of the weighing plate relative to the stationary environment.
For such a low-friction movement, the bearing device of the weighing plate may be designed in a cost-effective manner with at least one hydrostatic bearing.
According to a preferred embodiment, a bearing device of the weighing plate comprises a piezoelectric force transducer that may simultaneously function as a measuring device for determining forces on the weighing plate relative to the stationary environment.
The at least one measuring device for determining forces acting upon the vehicle is advantageously constructed with at least one load weighing cell for determining forces at the weighing plate parallel to the roadway plane. The aerodynamic longitudinal and side forces acting upon a vehicle can be determined by three such weighing cells whereby one of the cells determines the force of the movement of the weighing plate in the driving direction, and two additional cells each determine the forces of the weighing plate transversely to the driving direction.
Using the test bench according to the invention, aerodynamic lift forces can be determined on the vehicle to be measured. For the measuring of the lift forces, at least one weighing cell is provided by which forces can be determined in the direction of the vertical axis of the vehicle at the at least one contact surface between the vehicle and the weighing plate. According to the invention, the vehicle to be measured is advantageously fixed on the test bench by at least one fixing device on the running belt. The fixing device transmits the aerodynamic longitudinal and side forces to the weighing plate, where they can be measured by the above-mentioned weighing cells relative to the stationary environment. Furthermore, using the fixing device, the running belt is protected from slipping off-center and moving down.